(1) Field of the Invention
The present invention relates to a method and to a precoding circuit for minimising distortion of a signal being transmitted through a communication channel whereby the distortion is due to the transfer function of the communication channel. In particular, the invention relates to a method and a precoding circuit for pre-equalising a subset of communication channels where the first coefficient of the feedforward part of the transfer function is equal to zero, although the invention is not so limited.
(2) Description of Related Art
It is known to provide a Tomlinson-Harashima precoding circuit and method for pre-equalisation of a communication channel whose impulse response is finite and known, in other words, for communication channels with a feedforward transfer function which can be expressed by the following equation in the Z-Domain:H(z)=(1+a1.z−1+a2.z−2+a3.z−3+ . . . +an.z−n)
The Tomlinson-Harashima precoding method and circuit is disclosed in “New Automatic Equaliser Employing Modulo Arithmetic”, Electronic Letters, Vol. 7, Nos. 5/6, Mar. 25, 1971, pp. 138–139 by Tomlinson, M and Digital Communication, 2nd edition, by Edward A. Lee and David G. Messerschmidt, Kluwer Academic Publishers. FIG. 1 illustrates a block representation of such a precoding circuit according to Tomlinson-Harashima. An input data signal in the range
            +              M        2              ⁢                  ⁢    to    ⁢          -      M    2  is applied to a communication channel at the transmit end and is received at the receive end. In the Tomlinson-Harashima precoding circuit a feedback circuit is provided at the transmit end of the communication channel, and the feedback circuit has the transfer function which is the inverse of the feedforward part of the transfer function of the communication channel. A feedback signal from the feedback circuit is subtracted from the signal at the transmit end of the communication channel. A modulo operator is provided in the communication channel between the input and the output of the feedback circuit whereby a function kM is subtracted from the signal at the transmit end of the communication channel. The term k is an integer which may be positive, negative or zero and is chosen so that the output of the modulo operator is in the range
            +              M        2              ⁢                  ⁢    to    ⁢          -            M      2        ⁢                  .  . A similar modulo operator is provided at the receive end of the communication channel through which the signal at the receive end is passed for facilitating the recovery of the input data at the receive end. However, the Tomlinson-Harashima precoding circuit and method while it is suitable for pre-equalisation of a communication channel, it is only suitable for dealing with the feedforward part of the transfer function of the communication channel.
There is therefore a need for a method and a precoding circuit for pre-equalisation of a communication channel for minimising distortion of an input signal being transmitted in the communication channel which takes account of both the feedforward and feedback parts of the transfer function of the communication channel.